Centrifuges



Dec. 5, 1961 D. A. BOYLAND 3,011,695

CENTRIFUGES Filed July 13, 1948 49 EXTRACTION Pump DELI YER) PUMP FIG.4

United States Patent 3,011,695 CENTRIFUGES Donald Arthur Boyland, 24 Sheridan Gardens, Kenton, England Filed July 13, 1948, Ser. No. 38,529 Claims priority, application Great Britain July 23, 1947 Claims. (Cl.233-13) The present invention relates to centrifuges and is concerned with the Withdrawal of gases or vapours therefrom.

In centrifuges operating at high speeds, for example for the separation of gases of high molecular weight where the peripheral speed may be of the order of 4x10 cms. per second, the ratio of the pressure of gas at the periphery to that near the centre is very large. Thus, in an evaporative centrifuge, whereas pressure at the periphery may be of the order of 1000 mm. Hg, the pressure near the axis, where the concentration of the lighter fraction of the fluid is greatest, may be of the order of 0.001 mm. Hg or less. Difiiculty then arises in the withdrawal of the fluid from a region on or near the axis of the rotor at a rate adequate to maintain useful separation of the constituents.

The present invention is concerned with the provision of novel means for facilitating the withdrawal of fluid from the rotors of centrifuges.

The invention makes use of two facts:

(1) If a fluid (vapour or gas) is being centrifuged in a rotor, or in an end chamber fixed to the rotor, rotating at a high peripheral speed, the ratio of the pressure at or near the axis to that at the periphery of the rotor or chamber is much less if the fluid has a low or medium molecular weight than if it has a high molecular weight. For example, if the peripheral speed is 4x10 cms. per second and if the absolute temperature is 300 K., then with a gas of molecular weight 28 (nitrogen) the pressure ratio between the axis and periphery is 0.406, while for a heavy gas or vapour of molecular weight, say, 350, the pressure ratio is approximately 0.132

(2) The drift velocity of one gas through another is not very large. Thus if a heavy gas or vapour of molecular weight 350 is mixed with an equal volume of nitrogen, of molecular weight 28, and if the mixture is subjected to a centrifugal force of, say, 200,000 g., then if the pressure of the mixture is approximately 100 mm. absolute of mercury, the drift velocity of the heavy gas through the light gas will be of the order or 3 cms. per second.

According to the present invention, there is provided a method of extracting a relatively heavy fluid from the rotor of a centrifuge which comprises introducing a relatively light fluid through an inlet passage into a mixing chamber, which rotates With the rotor, to mix in this chamber with the relatively heavy fluid passing into the chamber from the interior of the rotor, and Withdrawing the lighter fluid from the mixing chamber through an outlet passage at a velocity greater than the drift velocity of the heavier fluid through the lighter fluid, whereby some of the heavier fluid is withdrawn with the lighter.

Further according to the invention, the rotor of a centrifuge is provided with a mixing chamber rotating therewith, a first passage through which a relatively heavy fluid to be extracted from the rotor can flow into the mixing chamber, an inlet passage by which a relatively light fluid can be introduced into the mixing chambet and an outlet passage by which the lighter fluid with an admixture of theheavier fluid can be withdrawn from the mixing chamber. The lighter fluid must be one which does not react chemically with the heavier fluid and preferably one which is readily separated from the heavier fluid. The inlet and outlet passages usually include radial portions and the cross-sectional area of such 'radial portions must be such that the velocity of the lighter fluid towards the axis is greater than the drift velocity of the heavier fluid through the lighter fluid away from the axis when the speeds of pumping the fluids have a suitable value.

The invention will be described by way of example with reference to the accompanying drawing in which FIGURE 1 is a view in cross-sectional elevation of a centrifuge rotor embodying the invention.

FIGURE 2 is an end view of part of an alternative form of end plate to that of FIGURE 1 used in a preferred embodiment of the invention,

FIGURE 3 is a View in cross-section on the line III- 111 of FIGURE 2,

FIGURE 4 is an enlarged view in cross-section on the line IV-IV of a part of FIGURE 2, and

FIGURE 5 is a fragment of the view of FIGURE 2 embodying a modification.

Referring to FIGURE 1, a centrifuge rotor 10 has an end plate 11 accommodating an end chamber divided into two parts 12 and 13 by a partition 14. The part 12 is enlarged at its periphery'to form an annular mixing chamber 15. The mixing chamber 15 connects with the axial region of the inside of the rotor 10 through passages 16 by which the lighter fraction of the relatively heavy centrifuged fluid can pass into the mixing chamber. Thus the passages 16 may connect with collecting bags or extraction chambers as described in the specification of co-pending Patent application No. 729,- 600 filed February 19, 1947. These extraction chambers have walls 27 and fluid enters them from the interior of the rotor in a region near the axis of the rotor through apertures 23 in the walls 27. The mixing chamber 15 is connected with the part 13 of the end chamber through passages 17. Two co-axial tubes 18 and 19 are provided on the axis of rotation 20. These may constitute, or may be mounted within, a driving spindle usually provided at one end of the rotor or a damping spigot usually provided at the other end thereof. The inner tube 18 connects with the chamber 12 whilst the annular passage 21 between the tubes 18 and 19 connects with the chamber 13. A delivery pump 29 is connected with the annular passage 21 and an extraction pump 30 is connected with the tube 18.

In operation a gas of relatively low molecular weight, for instance nitrogen, is introduced by the pump 29 into the mixing chamber 15 through the passage 21, chamber 13 and passages 17. The lighter fraction of a gas of relatively high molecular weight, for instance about 350, being centrifuged, enters the mixing chamber 15 through the passages 16 and the gases in the mixing chamber 15 are pumped out by the pump 30 through the chamber 12 and the tube 18.

The reason for the flow of the heavier gas with the the lighter through 12 and 13 will be understood from the following. The heavy and li ht gases mix in the chamber 15 but under no-flow conditions the partial pressure of the lighter gas near the axis 20 will be very much higher than that of the heavier gas, if the partial pressures are approximately equal in the mixing chamber 15. For example, using the figures given under 1) above, if the partial pressures of the two gases in the mixing chamber are both 500 mm. absolute of mercury, and if the peripheral speed of 4x10 cm. per second is that of the mixing chamber 15, the pressure of the lighter gas at the axis will be 0.406 500=203 mm. of mercury, while the pressure of the heavier gas at the axis will be 0.132X lO- 50O=0.0066 mm. of mercury.

Thus if the pressure in the tube 13 is maintained, by a suitable pump, at a pressure somewhat lower than the of rotation.

actress U pressure of the lighter gas in the passage 21, then the lighter gas will flow into the tube 15 and will be pumped away. Since, however, the heavier gas is mixed with the lighter one and since its speed of drift through the lighter gas is relatively low, some of the heaviergas will be drawn into the tube 1 3 and pumped away with the lighter gas, provided that a continuous supply of the lighter gas is circulated through from the passage 21 to the outlet through the tube 1% and provided that the rate of flow of the lighter gas is greater than the speed of drift. The cross-sectional area of the chamber 12 through which the mixture is drawn radially inwards is made such that the required rate of flow is obtained at the desired pumping speed.

The partial-pressure of the lighter gas in the mixing chamber 15 should not be so high as to impede the flow of the heavier gas through the passages 16 or to allow any appreciable quantity of thelighter gas to enter the rotor through these passages.

though not essential.

The effect of the U tube is that a plug of the heavier gas is provided at the part 25 of the U tube which is radially outermost. because the pressure of the heavier gas rises steeply with increase of distance from the axis The pressure of the lighter gas, however, increases much more slowly with increase of distance from the axis. Consequently although the pressures of the two gases may be nearly the same in the mixing chamber 15', the pressure of the heavier gas is considerably greater than that of the li hter gas in the passage 25. It is, therefore, difiicult for any lar e quantity of the lighter gas to ditfuse through the passage 25.

It has been found advantageous, inorder to improve stability, so to shape the cross-section of the supply tubes 22, for example by a constriction in these tubes, that a small pressure drop, for instance 5 to mm. Hg, is maintained in these tubes under normal working conditions;

The cross-section of the U tube may also be made relatively small, particularly at 25, so that the heavier gas passes through it at a fairly high velocity against the direction of flow of any of the lighter gas which may tend to pass the rotor.

When fast pumpin -out of the mixture from the mixing chamber is desired, the outlet passages 23 are preferably shaped to follow a smooth curve from the mixing chamber 15' to the tube 13, the curvature being in a plane transverse with respect to the rotor axis; that is to say these passages would appear curved in FIGURE 2. The curvature is made such that the convex side faces in the direction of rotation as indicated at 23 in FIGURE 5 assuming the direction of rotation to be anti-clockwise. In this way advantage can be taken of the dynamic head pressure of the rotating fluid passing through the passage 23' as its speed of rotation falls in passing from near the periphery to nearthe axis of the rotor, in order to assist extraction. Similarly the inlet tubes 22 may be curved as indicated at 22' in FIGURE 5.

' posite end of the rotor connect with the interior of the rotor radially outside the extraction chamber walls 27. The method of the invention can be applied at both ends. if a fluid to be centrifuged is introduced into the rotor continuously, this arrangement permits continuous withdrawal of a lighter fraction from a passage 16 as shown in FIGURE 1 and of a heavier fraction from a passage connecting with the rotor outside the chamber walls 27.

I claim:

1. A method of extracting a relatively heavy fluid from the rotor of a centrifuge, the rotor having a mixing chamber rotatable therewith, a connection between the mixing chamber and the interior of the rotor for the passage of the relatively heavy fluid from the rotor to the mixing chamber, and inletand outlet passages connecting with the mixing chamber, the method comprising introducing a fluid which is much lighter than said relatively heavy fluid through said inlet passage into said mixing chamber to mix with the relatively heavy fluid from said rotor, and pumping the lighter fluid from the mirn'ng chamber through the outlet passage in a direction having at least a component which is radially inward at a velocity greater than the drift velocity of the heavier fluid through the ligher fluid, whereby some of the heavier fluid is withdrawn with the lighter.

2. A method of extracting a relatively heavy fluid from the rotor of a centrifuge, the rotor having a mixing chamber rotatable therewith, a connection between the mixing chamber and the interior of the rotor for the passage of the relatively heavy fluid from the rotor to the mixing chamber, and inlet and outlet passages connecting with the mixing chamber, the method comprising introducing a relatively light fluid in a radially outward direction through said inlet passage into said mixing chamber to mix with the relatively heavy fluid from said rotor, and pumping the lighter fluid from the mixing chamber in a radially inwardvdirection through the outlet passage at a velocity greater than the drift of the heavier fluid through the lighter fluid, whereby some of the heavier fluid is withdrawn with the lighter.

3. A centrifuge having a rotor comprising a mixing chamber, a first passage for conveying a relatively heavy gas from the interior of said rotor to said mixing chamber, an inlet passage, means for introducing a relatively light gas into said mixing chamber through said inlet passage, an outlet passage connecting with said mixing chamber, and an extraction pump connected to said outlet passage for extracting a mixture of said relatively heavy and relatively light gases from said mixing chamber through said outlet passage, said outlet passage comprising a tubular portion extending inwards towards the axis of rotation of said rotor in a smooth curve lying at least approximately in a lane transverse with respect 7 to the said axis.

4. A centrifuge having a rotor comprising a mixing chamber, a first passage for conveying a relatively heavy fluid from the interior of said rotor to said mixing chamber, said passage including a U-tube having its inlet and outlet at a shorter radius of rotation than its base, an inlet passage, means for introducing a relatively light fluid into said mixing chamber through said inlet passage and an outlet passage for the extraction of a mixture of said relatively heavy and relatively light fluids from said mixing chamber.

5. A method of extracting a relatively heavy gas from the rotor of a centrifuge, the rotor having a mixing chamber rotatable therewith, a connection between the mixing chamber and the interior of the rotor for the passage of the relatively heavy gas from the rotor to the mixing chamber, and inlet and outlet passages conmeeting with the mixing chamber, the method comprising introducing a gas, which is much lighter than the said relatively heavy gas and which issubstantially inert chemically in relation to the relatively heavy gas, through said inlet passage into said mixing chamber in a direction having at least a component which is radiaily inward to mix with the relatively heavy gas, and pumping the lighter gas from the mixing chamber through the outlet passage at a velocity greater than the drift velocity of the heavier gas through the lighter gas, whereby some of the heavier gas is withdrawn with the lighter.

6 References Cited in the file of this patent UNITED STATES PATENTS 1,649,118 Jones Nov. 15, 1927 2,107,035 Hall Feb. 1, 1938 FOREIGN PATENTS 381,913 Great Britain Oct. 13, 1932 

